1. Field of the Invention
The present invention relates to x-ray tubes, and more particularly, to a high power x-ray tube that produces an imaging spot size that is continuously adjustable over a given range.
2. Description of Related Art
It is well known in the art to use a source of x-rays to produce planar images for medical and technical diagnostic applications. In the field of technical diagnostic imaging, x-rays are especially effective at penetrating internal structures of a solid imaging object, and the images formed by the x-rays that pass therethrough reveal internal flaws or structural defects of the object. Technical diagnostic x-ray imaging thus provides a valuable quality control inspection tool for evaluating structural aspects of a product during manufacture and over the useful life of the product. This form of diagnostic analysis is advantageous over other types of evaluation, since the imaging object need not be destroyed in the process of the evaluation. For this reason, technical diagnostic imaging is also known as non-destructive testing.
A x-ray tube for technical imaging applications typically comprises an electron gun having a cathode that is excited to emit a beam of electrons that are accelerated to an anode. The anode may be comprised of a metal target surface, such as tungsten, from which x-rays are generated due to the impact of the accelerated electrons. By disposing the anode surface at an angle to the axis of the electron beam, the x-rays may be transmitted in a direction generally perpendicular to the electron beam axis. The x-rays may then be passed through a beryllium window used to provide a vacuum seal within the x-ray tube. Thereafter, the x-rays exit the x-ray tube along a generally conical path where the apex of the cone is roughly coincident with the spot on target formed by the impinging electron beam.
The amount of magnification provided by an x-ray tube is dependent, in part, upon the spot size, which is sometimes referred to as the imaging spot size. A smaller spot size typically enables greater magnification while maintaining desirable image sharpness, but covers a smaller portion of the imaged object. This is accomplished, for example, by situating the imaged object closer to the x-ray source, that is the x-ray imaging spot, with respect to the position of the photographic film or other x-ray image recording means. Conversely, a larger spot size can image a greater portion of the imaged object, but typically at a lower magnification level. In this case, in contrast to the smaller spot size, the area of electron beam impingement is larger on target; hence, a higher voltage, higher current, or higher voltage and current electron beam can be utilized without thermally overstressing the target. Conventional x-ray tubes are typically limited to providing either a single spot size, or in some cases, two discrete spot sizes. To provide two different spot sizes, the x-ray tubes have two distinct cathode filaments that are alternatively energized to provide electron beams of different diameters. An operator of an x-ray tube will select one of the cathode filaments depending upon the desired magnification level and size of the imaging object. A drawback of such systems is that the spot size of the x-ray tube cannot be optimized for a particular imaging operation.
In conventional x-ray tubes, another approach to reducing the effective spot size is to position the anode surface at an angle flatter than 45.degree. to the beam axis while maintaining the x-ray output cone oriented at 90.degree. to the beam axis. An advantage of this approach is that the flat anode angle lowers the power density on the anode, which, if excessive, can cause undesirable melting and vaporization of the tungsten target material. Moreover, to geometrically compensate for the flat anode angle, the electron gun is configured to provide an elliptical electron beam so that the x-ray spot will have a circular cross-section. This lack of axial symmetry of the electron gun can add cost and complexity to the manufacture of the x-ray tube. Further, the electron beam spot is rarely elliptical, and the resultant x-ray imaging spot is usually distorted in shape, has intensity irregularities, and is non-circular leading to inferior quality x-ray images.
Thus, it would be desirable to provide an x-ray tube having a spot size that is continuously adjustable over a given range to allow greater flexibility in the imaging operations. It would also be desirable to provide an x-ray tube constructed with an axially symmetric geometry to simplify manufacture and improve the symmetry and intensity of the x-ray spot. A further desirable advantage is that the spot size and x-ray intensity can be varied without repositioning the object. Finally, it would be desirable to provide an x-ray tube having a more uniform intensity circular x-ray imaging spot for improved quality x-ray images.